This invention relates to a method and an apparatus for molten salt electroplating of steel members. More particularly, it relates to a method and an apparatus for molten salt electroplating whereby Al plating having superior adhesion to a base metal can be formed.
Up to the present time, although various methods of forming Al plating on steel members (steel strip, steel sheet, steel wire, etc.) by molten salt electroplating have been known, these methods have been put to almost no actual use.
An example of a conventional molten salt electroplating apparatus for forming Al plating is illustrated in FIG. 1. This example will be explained for the case in which the steel member being electroplated is a steel strip.
As shown in this figure, the front end of a steel strip 2 which is unwound from a pay-off reel 1 is welded to the rear end of the previous steel strip by a welding machine 3, after which it passes through a looper 4 and is sent to a pretreatment apparatus 5. Here, it is subjected to necessary pretreatments such as degreasing and preheating prior to electroplating. Next, its surface is plated in an electroplating tank 6, after which it is washed in a washing tank 7 and then dried in a drier 8. It then passes through a looper 9 and a shearing machine 10 and is wound onto a tension reel 11.
The electroplating tank 6 is filled with an electroplating solution 12. Plating is formed on the surface of the steel strip 2 as it passes between electrodes 13 which are disposed in the electroplating solution.
Electroplating using the above-described method has not been performed industrially to any great extent for the following reasons.
At present, industrial electroplating is largely electrocrystallization from an aqueous solution. While there are roughly 30 different types of metals which can be electroplated by electrocrystallization from an aqueous solution, there are approximately only 10 types on which such electroplating is actually performed. Accordingly, the other types must be plated using a nonaqueous solution or a molten salt bath. While electrocrystallization in a molten salt bath can be performed on almost every metal, the condition of the plating is generally poor with the deposited metal being in the form of dendrite crystals or a powder, so the adhesion of the plating to the base metal is poor. Therefore, it is difficult to obtain smooth plating by molten salt electroplating, and furthermore, molten salt electroplating is difficult to perform.
The surface of a steel member to be plated is activated by wet treatments such a washing in acid followed by washing with water. Then, prior to electroplating, it is necessary to dry the steel member because if an water is introduced into the molten salt electroplating bath, the bath will rapidly deteriorate. There must be strict control of both moisture and the atmosphere of an electroplating line. However, if drying is performed by heating to a high temperature (such as 80.degree. C.) in air, the adhesion of the plating which is subsequently formed by molten salt electroplating greatly decreases.
In addition, although salt adhering to the surface of a steel member which was plated is washed off in the washing tank 7 of FIG. 1, when the salt which is used in Al plating is washed off using water, the salt reacts with the water and can not be reused in the electroplating bath. Accordingly, salt which adheres to the steel strip 2 and is removed from the electroplating tank 6 ends up being discarded, and the running costs of a steel strip electroplating line of this type are increased. Furthermore, as the waste water from the washing tank 7 contains a large amount of salt, it is necessary to employ largecapacity water treatment equipment.
Another problem is that molten salt vapor is present in the empty space within the electroplating tank 6 above the electroplating solution 12. If this vapor were to leak out of the electroplating tank 6, it would be harmful to humans, so the inside of the electroplating tank 6 must be ventilated and kept at a pressure below atmosphere pressure. The gas which is exhausted from the electroplating tank 6 is passed through a spray tower 14 and after gaseous salt in the exhaust gas is removed, the gas is exhausted into the atmosphere by an exhaust fan 15.
If the gaseous salt does not react with water, the salt can be recovered by drying the treatment liquid used for cleaning the exhaust gas. However, if the gaseous salt reacts with water, the salt can no longer be recovered. Therefore, the salt which is removed from the electroplating tank 6 as exhaust gas ends up being discarded, and the running costs of this method are further increased.
Furthermore, an increase in the amount of salt in the treatment liquid produces a need for larger water treatment equipment and leads to an increase in treatment costs.